Hotlinking is NEVER acceptable behavior unless it’s from a site that promotes it (like imageshack.us) as something they provide.

Unless you want to figure out a way to have your forum dissallow links TO the image site, the best thing to do is make a post now to tell people not to hotlink. And when they do it, edit the post to remove the image and tell them ‘Hotlinking is BAD AND WRONG.’

There aren’t any system to prevent users to hotlink images. I’ve managed a forum with more than 50 thousands unique visitors per day and in that situations is absolutely imposible. The only way is to have a lot of moderators

But this would cause issues with peoples own hosting accounts, and also linking to images from manufacturers who would want their images linked on forums. It is a cycling site and many discussions revolve around new bikes – it would be a huge list.

I would say any perfectly acceptable site, which is OK with hotlinking, would NEVER do this. If you’re using their site against their terms of use (i.e. no hotlinking allowed) then they’re within their rights to serve whatever image they feel like.

I think if I had this problem I would tell my users that images will only be displayed from a whitelist of image hosting companies; no other companies will be allowed. Then, whitelist that site only. If it’s a perfectly acceptable/reputable site, which allows hotlinking, then they will never replace the image and serve something else. Of course, your users should not be hotlinking in the first place, and because you allow it, you have this problem. Allowing uploads to your server with the bb attachment plugin eliminates the issue of images being replaced and displayed to your members. You mentioned copyright issues: well, if your members don’t have the right to use the image in the first place, they shouldn’t be loading it onto an image hosting service anyway: the copyright problem exists there as well.

I suspect you could start out with a small whitelist or blacklist and see 80% of the problem go away very quickly. There’s probably a “good enough” solution. Do something, don’t overthink it.